Method for making an airtight, flexible cell for enclosing electrical switching apparatus in the application of a door edge safety strips is disclosed. The illustrated technique describes a method which includes a series of steps which may be applied to the raw sheet material used in producing the airtight cell which reduces the number of steps necessary to encase the switching apparatus within the airtight cell, simplifying the folding operations required to produce a seam to enclose the switching apparatus. The method provides for the use of three independent sheets of material which are ultimately fastened to each other as described using radio frequency heating apparatus to provide the fastening means of the material used to create the airtight cell. A method of manufacturing airtight cell in the prior art is shown in Miller, U.S. Pat. No. 4,487,648, in which is illustrated a technique for a series of steps which are applied to raw sheet material which reduced the number of folding operations required to produce a seam. It would be appreciated by review of such prior art that an improvement consists of the shape alteration of the sheet of material used to form the end of one side of the cell along its longitudinal axes. The improved shape reduces the number of steps necessary to fasten a seam by eliminating one step.